There are a wide variety of metal oxides and such metal oxides are used for various applications. Indium oxide is a well-known material and has been used for transparent electrodes required in liquid crystal display devices or the like.
Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics are tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. Thin film transistors in which a channel formation region is formed using such metal oxides are already known (e.g., see Patent Documents 1 to 4, Non-Patent Document 1, and the like).
As metal oxides, not only single-component oxides but also multi-component oxides are known. For example, InGaO3(ZnO)m (m: natural number) having a homologous phase is known as a multi-component oxide semiconductor including In, Ga, and Zn (e.g., see Non-Patent Documents 2 to 4 and the like).
Furthermore, it has been proved that an oxide semiconductor including such an In—Ga—Zn-based oxide is applicable to a channel formation region of a thin film transistor (e.g., see Patent Document 5, Non-Patent Documents 5 and 6, and the like).
In order to achieve high speed operation of a transistor or the like, miniaturization of the transistor is needed. For example, in Patent Document 6, a thin film transistor including an oxide semiconductor used for a channel layer with a thickness of about 10 nm is disclosed. In Non-Patent Document 7, a thin film transistor including an oxide semiconductor whose channel length is 2 μm to 100 μm is disclosed.